An organic EL device is a spontaneous light emitting device which utilizes such a principle that a fluorescent substance emits light by virtue of recombination energy of holes injected from an anode and electrons injected from a cathode by an application of an electric field. Since an organic EL device of the laminate type capable of being driven under low electric voltage has been reported by C. W. Tang et al. of Eastman Kodak Company (see Non Patent Literature 1), many studies have been conducted for an organic EL device using an organic material as a constituent material.
Tang et al. discloses an organic EL device having a laminate structure in which tris(8-quinolinolato)aluminum is used in a light emitting layer and a triphenyldiamine derivative is used in a hole transporting layer. Advantages of adopting the laminate structure in the organic EL device include: (i) injection efficiency of holes into the light emitting layer can be increased; (ii) efficiency of forming excitons which are formed through recombination in the light emitting layer can be increased by blocking electrons injected from the cathode toward the light emitting layer in the hole transporting (injecting) layer; and (iii) excitons formed in the light emitting layer can be easily enclosed in the light emitting layer. In order to increase the efficiency of recombination of injected holes and electrons in the organic EL device having such laminate structure, there have been made refinements of the device structure and a method of forming the device, and studies on a material itself for each layer.
In general, when an organic EL device is driven or stored in an environment of high temperature, there occur adverse affects such as a change in luminescent color, a decrease in luminous efficiency, an increase in driving voltage, and a decrease in lifetime of light emission.
In order to prevent such adverse effects, there have been reported, as hole transporting material, aromatic amine derivatives each having a carbazole skeleton (see Patent Literatures 1 to 3), an aromatic amine derivative having a dibenzofuran skeleton or a dibenzothiophene skeleton and a fluorene skeleton (see Patent Literature 4), and the like.